Resistance Temperature Sensor

ABSTRACT

A resistance temperature sensor with a first temperature sensor element and a second temperature sensor element, wherein the first temperature sensor element comprises a first measuring path and the second temperature sensor element a second measuring path, wherein the first and the second measuring paths extend on a substrate, wherein the substrate has an anisotropic thermal expansion with at least two mutually differing expansion directions (a, c), and wherein a projection of the first measuring path on the expansion directions (a) differs from a projection of the second measuring path on the expansion directions (c).

CROSS-REFERENCE TO RELATED APPLICATION

This is a continuation of U.S. patent application Ser. No. 13/166,854,filed on Jun. 23, 2011, which is a nonprovisional application, claimingthe benefit of U.S. Provisional Application No. 61/344,285 filed on Jun.23, 2010.

TECHNICAL FIELD

The invention relates to a resistance temperature sensor and a measuringdevice with such a resistance temperature sensor.

BACKGROUND DISCUSSION

Temperature dependent resistors, or resistances, arrangements comprisinga number of such resistors and measuring devices using such resistorsfor registering a process variable, especially temperature, are known inthe state of the art.

Thus, for example, Offenlegungsschrift EP 0828146 A1 discloses aself-monitoring temperature measuring apparatus with a first and asecond resistance element with positive or negative resistancecoefficients. The two resistance elements lie in parallel electricalcurrent paths, of which one includes a diode which allows electricalcurrent to flow in one direction only. The resistances of the tworesistance elements are determined by a corresponding circuit whichperiodically changes a voltage applied to the electrical current paths,especially by reversing polarity.

An arrangement of sensor elements is also known from the GebrauchsmusterDE 202004021438 U1. In such case, the sensor elements have electricalimpedances that differ with regard to their temperature coefficientsand, integrated in a sensor head, are thermally coupled with one anotherand with the medium to be measured.

A resistance temperature sensor, which comprises a first and a secondsensor unit, which, for example, are manufactured using thin filmtechnology, wherein the sensor units are arranged in parallel planes oneon top of the other in order to enable a compact construction of theresistance temperature sensor, is known from Offenlegungsschrift DE102006005393 A1

However, these arrangements require complex wiring as well as anincreased space requirement due to the separately executed temperaturesensors, or at least are not directly suitable for self monitoringand/or self calibrating. Moreover, measuring devices, which areinstalled, for example, in a plant utilizing process automationtechnology, should not disturb the actual process. Consequently, furtherminiaturization is desired. Also simplification is desired, in order toreduce costs, for example, of manufacture of the measuring devices.

SUMMARY OF THE INVENTION

An object of the invention is to provide a compact apparatus fortemperature measurement not having the above mentioned disadvantages.

According to the invention, the object is achieved by a resistancetemperature sensor as well as a measuring device with such a resistancetemperature sensor.

As regards the resistance temperature sensor, the object is achieved bya resistance temperature sensor having a first temperature sensorelement and a second temperature sensor element, wherein the firsttemperature sensor element has a first measuring path and the secondtemperature sensor element has a second measuring path, wherein thefirst and the second measuring paths extend on a substrate, wherein thesubstrate has an anisotropic thermal expansion with at least twomutually differing expansion directions, and wherein a projection of thefirst measuring path on the expansion directions differs from aprojection of the second measuring path on the expansion directions.

Instead of the substrate, any other support element with an anisotropicthermal expansion can also be used. By using two different temperaturesensor elements, it can be assured that drift occurring, in given cases,in the resistance temperature sensor can be recognized and/or diagnosed.Moreover, in the case of failure of one of the temperature sensorelements, the determining of the temperature can be continued using theother temperature sensor element. The first or the second temperaturesensor element can thus be applied for monitoring the second or thefirst temperature sensor element.

Via the first or the second measuring path, a value and/or curve of aphysical variable, here the temperature, can be ascertained. The presentinvention accordingly provides differently extending measuring paths fordetermining temperature. Furthermore, the present invention providesthat the measuring paths can extend on a substrate with an anisotropic,i.e. directionally dependent, thermal expansion. For such purpose, thesubstrate can have at least two directions having different thermalexpansions. Thus, the first and the second temperature sensor elementcan comprise the same substrate for example, but differ, however, asregards the measuring paths which extend along the substrate. In thisway, the first and the second measuring paths also differ as regardstheir projections on the expansion directions. For example, themeasuring paths can extend so that the components of the measuring pathsdiffer from one another as regards the directions predetermined by theexpansion directions. In this way, the measuring paths thus experience athermal expansion due to the underlying substrate with an anisotropicthermal expansion, and the resistances present over the respectivemeasuring paths change. It can additionally be exploited that theresistance of the first measuring path and the second measuring path donot change at the same rate due to the substrate with an anisotropicthermal expansion, since for example, the coefficient of thermalexpansion of the substrate is directionally dependent and theresistances differ from one another depending on direction. This canessentially be due to the thermal expansion of the substrate. Thesubstrate can thus have, for example, a directionally dependent lengthcoefficient of expansion or a directionally dependent volume coefficientof expansion. The size of this effect, i.e. the thermal expansion, can,in such case, depend on the material used for the substrate. The thermalexpansion or the coefficient of thermal expansion can also betemperature dependent.

If one assigns, for example, a first vector to one of the expansiondirections, and, for example, a second vector to the first measuringpath, then the projection of the second vector on the first vector isgiven by the vector that extends in the direction of the first vector,as limited by the foot of the perpendicular to the first vector, whichperpendicular extends through the end point of the second vector. Ofcourse, a plurality of vectors which describe, for example the route ofthe first or second measuring path, can also be projected onto theexpansion directions. This projection, or also only the length of theprojected vector, can be taken into consideration for a comparison fordetermining whether the first and the second measuring paths differ fromone another as regards their projections on the expansion directions.

In an embodiment of the resistance temperature sensor, the first and/orthe second temperature sensor element comprise at least one thin filmcoating which is applied to the substrate. In such case, the thin filmcoating can be applied to the substrate by a conventional method knownfrom the state of the art, for example by means of a physical and/orchemical gas deposition process. The thicknesses of the thin film layercan, in such case, lie in the micrometer range (μm), especially it canalso be less than 1 μm (10⁻⁶ m). Thin film coatings should not be onlyunderstood as coatings which are produced by additive processes such assputtering, for example, but also coatings which arise by subtractiveprocesses such as etching, for example.

In an embodiment of the resistance temperature sensor, the thin filmcoating forms a thin film resistance. Thus, especially a single,especially a continuous thin, film coating, on which two measuring pathsare defined, can be utilized. In this way, a compact resistancetemperature sensor can be produced which can additionally calibrateand/or monitor itself. In such case, the effect that in the case of anexpansion of the substrate, the thin film coating applied to thesubstrate likewise expands or shrinks and thereby changes the electricalresistance of the first and the second measuring path, is exploited.

In another embodiment of the resistance temperature sensor, the firstmeasuring path is a first thin film resistance and the second measuringpath is a second thin film resistance, wherein the first and the secondthin film resistances are applied on different surface regions of thesubstrate. Accordingly, the first measuring path can also be applied toa thin film layer, especially in different surface regions of thesubstrate than the second measuring path. For example, the thin filmresistances and the associated measuring paths and the associatedtemperature sensor elements can be applied on opposite sides of thesubstrate.

In an embodiment of the resistance temperature sensor, the first or thesecond measuring path extends on the substrate, such that the firstmeasuring path experiences a different thermal expansion than the secondmeasuring path due to the anisotropic thermal expansion of thesubstrate. For example, the substrate can experience a thermally relatedcontraction in one direction and a thermally related expansion inanother direction. This can also affect then the first and the secondmeasuring path or the thin film coating applied to the substrate in themanner mentioned above.

In an embodiment of the resistance temperature sensor, the firstmeasuring path extends on the substrate at least sectionally along anexpansion direction which has a different thermal expansion compared toan expansion direction along which the second measuring path extends.

In an embodiment of the resistance temperature sensor, at least a firstand a second pair of electrical contacts are provided, by means of whichthe first and/or the second measuring path are contactable. One of thetemperature sensor elements can thus essentially comprise the substrate,the thin film coating applied thereon, the measuring path defined by thethin film coating as well as the contacts for contacting the thin filmlayer. The resistance temperature sensor provided can then comprise atleast two, preferably exactly two, such temperature sensor elements. Thefirst or second measuring path extends, in such case, between the firstor the second pair of electrical contacts.

In an embodiment of the resistance temperature sensor, the first and,respectively, the second measuring paths are predetermined by the firstand, respectively, the second pair of electrical contacts. The measuringpaths can be defined by the positioning of the contacts on the thin filmcoating. Thus the first pair of electrical contacts by which the firstmeasuring path is defined, for example, can be arranged on opposite endsof the thin film coating. Likewise the second pair of contacts can bearranged on opposite ends of the thin film coating. Moreover, thecontacts can be so arranged, that, for example, an imaginary connectingline between the second pair of electrical contacts and, for example, animaginary connecting line between the first pair of contacts form anangle a (alpha), wherein the angle a is preferably selected as afunction of the expansion directions of the anisotropic substrate, andlies especially preferably between 20° and 160°. For example, theimaginary connecting lines can in such case preferably match theexpansion directions of the substrate with anisotropic thermalexpansion.

In an embodiment of the resistance temperature sensor, the electricalcontacts are arranged on the substrate so that the first measuring pathexperiences a different thermal expansion than the second measuringpath.

In an embodiment of the resistance temperature sensor, the electricalcontacts are provided, in each case, on essentially opposite ends of atleast one thin film coating.

In an embodiment of the resistance temperature sensor, the contactsadjoin a single thin film coating applied on the substrate.

In an embodiment of the resistance temperature sensor, the contactsadjoin different thin film coatings, especially ones separated from oneanother, on the substrate.

In an embodiment of the resistance temperature sensor, the first and/orthe second thin film coating has a thickness between 0.5 μm and 10 μm.

In an embodiment of the resistance temperature sensor, the substrate hasa thickness between 300 μm and 2 mm.

In an embodiment of the resistance temperature sensor, the substrate hasa first expansion direction a, in which one thermal expansion occurs,wherein the substrate has a second expansion direction c, in which onethermal expansion occurs.

In an embodiment of the resistance temperature sensor, the thermalexpansion in the second expansion direction c is smaller than thethermal expansion in the first expansion direction a.

In an embodiment of the resistance temperature sensor, a thermallyrelated expansion occurs along the expansion direction a of thesubstrate and a thermally related contraction along the expansiondirection c of the substrate.

In an embodiment of the resistance temperature sensor, the first and/orthe second thin film coating experiences an expansion along theexpansion direction a and a contraction along the expansion direction cdue to the thermal expansion of the substrate.

In an embodiment of the resistance temperature sensor, at least one thinfilm coating comprises a single material.

In an embodiment of the resistance temperature sensor, the materialwhich forms at least one thin film coating has essentially the samethermal resistance and, respectively, expansion coefficient.

In an embodiment of the resistance temperature sensor, the substrate isan anisotropic crystalline material.

In an embodiment of the resistance temperature sensor, the substrate isessentially anisotropic beta-eucryptite, LiAlSiO4 or a lithium aluminumsilicate.

In an embodiment of the resistance temperature sensor, the substrate hasa rectangular, prismatic, ellipsoidal or circular shape.

Regarding the measuring device, the object is achieved by a measuringdevice for determining temperature with a resistance temperature sensoraccording to one of the preceding embodiments.

In an embodiment of the measuring device, the first and the secondmeasuring paths serve to determine the ambient temperature.

In an embodiment of the measuring device, the resistance measurements ofthe first and the second measuring paths serve for diagnosis of theresistance temperature sensor or the measuring device.

In an embodiment of the measuring device, the measuring device has acontrol/evaluation unit available, which serves to compare the measuredresistances of the first measuring path and the second measuring pathwith each other

In an embodiment of the measuring device, the measuring device includestwo measurement signal inputs, which serve to connect the firsttemperature sensor element and the second temperature sensor element tothe control/evaluation unit, for example one integrated in a transmitterunit.

Additionally, the object can be achieved by a corresponding method forthe manufacture and/or operation of a resistance temperature sensor or ameasuring device.

Another embodiment of the invention provides that, by means of the firstand the second temperature sensor element, in each case, an informationportion is ascertained, from which the process variable can then bedetermined totally. The process variable can be, for example, thedeviation of the measurement signal of the first temperature sensorelement from the measurement signal of the second temperature sensorelement, as would be used in a calibration, for example.

Additionally, the resistance temperature sensor and/or at least thefirst and/or the second temperature sensor element can serve and/or beoperated as a heating element. Then, for example, the anisotropicexpansion of the substrate can be utilized in order to calibrate theresistance temperature sensor. In such case, the resistance temperaturesensor can also additionally serve as a heating element of a thermal,flow measuring device.

BRIEF DESCRIPTION OF THE DRAWINGS

The invention will now be explained in greater detail on the basis ofthe appended drawing, the figures of which show as follows:

FIG. 1 is a plan view of a resistance temperature sensor according tothe state of the art;

FIG. 2 is a plan view of a resistance temperature sensor in anembodiment of the present invention, wherein the substrate hasanisotropic thermal expansion;

FIG. 3 is a resistance temperature sensor in an additional embodiment ofthe present invention, wherein the measuring path meanders;

FIG. 4 is a resistance temperature sensor in an additional embodiment ofthe present invention with a likewise meandering measuring path;

FIG. 5 is a schematic representation of the crystalline structure of ananisotropic substrate at normal temperature;

FIG. 6 is a schematic representation of the crystalline structure of theanisotropic substrate at temperature increased relative to normaltemperature;

FIG. 7 is a schematic representation of a cross section through anembodiment of the proposed invention, wherein the thin film resistor issurrounded by an inerting embedding material and a substrate cover;

FIG. 8 is a schematic representation of a cross section through anotherembodiment of the proposed invention, in which the thin film resistor issurrounded only by an inerting, embedding material;

FIG. 9 is a schematic representation of two measuring transducers, whichare connected to a transmitter unit;

FIG. 10 is a schematic representation of a measuring transducer with abuilt-in transmitter unit; and

FIG. 11 is a schematic representation of a device architecture in anindustrial plant.

DETAILED DESCRIPTION IN CONJUNCTION WITH DRAWINGS

FIG. 1 shows a resistance temperature sensor according to the state ofthe art. In such case, a thin film layer 2, which is contactable viaelectrical contacts 31, is applied on a substrate 1. A measuring path onthe thin film layer 2 is defined by the contacts 31. The measuring pathextends, in such case, between the contacts 31. The measuring path has aso-called thin film resistance. The measuring path and the so-calledthin film resistance are subject, in such case, to the thermal expansionof the underlying substrate 1.

A thin film resistance is generally a type of resistance used forintegrated circuits, for example, and is embodied as a thin layer ofresistive material. Numerous resistive materials can be used for formingthin film resistances. The behavior of such thin film resistances isdefined by a number of parameters, which include the resistance, theresistance tolerance and the temperature coefficient of resistance (TCR)(measure of resistance change with temperature change).

The resistance in FIG. 1 is measured with four conductor technology,i.e. an electrical current flows between two of the connection lines 5while only a voltage is sensed between the other two connection lines 5,which are thus essentially free of electrical current.

The substrate 1 applied in the example of an embodiment according toFIG. 1 has an isotropic thermal expansion, so that, in the case of atemperature change, the expansion of the material occurs independentlyof spatial orientation.

FIG. 2 shows a resistance temperature sensor 10 according to anembodiment of the present invention. In such case, the resistancetemperature sensor 10 serves to measure temperature and includes a thinfilm coating 2 with a thickness between 0.5 and 10 micrometers made fromcoating materials containing conductive metal, transition metal, carbon,or carbon nano tubes, which are applied on a plate-like substrate 1 witha thickness between 300 micrometers and 2 mm, wherein the substrate 1has anisotropic thermal coefficients of expansion, wherein theanisotropic substrate surface has one principal direction a, in which anincreased thermal expansion occurs in a direction a′, and in thedirection c perpendicular to a and lying in the same plane, in the caseof heating, a contraction occurs in the direction c′ or else a smallerthermal expansion compared with the direction a, whereby the bonded thinfilm coating experiences an expansion in direction a′ and a contractionin direction c′.

Furthermore, the resistance temperature sensor 10 can have one or morethin film segments, for example of the same coating material 2 with thesame thermal resistance coefficient, which are applied, in each case, inthe orientation a and in the orientation c to the same anisotropicsubstrate 1. The thin film coating 2 shown in FIG. 2 is contacted onopposite ends in the orientations a and c with two or more electricalconnections in the form of electrical contacts 6, 7. Via the measuringpaths thus formed, resistance measurements through the contacts 6 atdifferent coating sections of the same coating material 2 with the samethermal resistance coefficient on an anisotropic substrate 1 can be usedfor a measurement difference evaluation. The substrate 1 applied forthis purpose can be an anisotropic crystalline material, e.g.anisotropic crystalline β-eucryptite, LiAlSiO4 or a lithium aluminumsilicate. The substrate 1 for this purpose can also have a rectangular,prismatic, ellipsoidal or circular, plate-like shape. Thus, a substratewith an essentially circular outline is shown in FIG. 3 and one with anessentially square outline in FIG. 4. In general, the substrate 1 can becomposed of at least one anisotropic material that has a negativethermal expansion at least in one principal direction. The conductivetraces 72, especially flat conductive traces, can be covered U-shaped bythe substrate 1, as shown in FIG. 7 and FIG. 8, wherein the substrate,in the latter case, extends laterally beyond the conductive traces 72.The conductive trace(s) 72 can also be completely surrounded by theanisotropic substrate material 71, as in FIG. 7, wherein the substratebase part 71 accommodating the conductive trace 72 is covered by anadditional substrate flat part 3, which has an anisotropic orientationof the same sense as the base material 71. On the other hand, theconductive trace 2 can simply be “capped” with an embedding material asshown in FIG. 8.

Moreover, both surfaces of the anisotropic substrate 1 can be equippedwith conductive traces 2, 32, 42.

As shown in FIG. 3 and FIG. 4, the coating can comprise meandering,hairpin curved conductive traces 32, 42. The conductive traces 32, 42can have a rectangular cross section or an ovally rounded off, crosssection.

The resistance temperature sensor 10 can be assembled of many layers,wherein a multilayered, sandwich-type construction is composed of planarportions of anisotropic substrate 1 and a conductive thin film coatings2, which are enclosed by an inerting, dielectrically insulating,embedding material 4.

FIG. 5 and FIG. 6 show a schematic representation of the crystallinestructure of the material, which forms the substrate 1. The substrate 1has, in such case, two principal expansion directions, along which thesubstrate 1 experiences a change of length in the case of a temperaturechange. In direction a, the substrate experiences an expansion whileexperiencing a contraction in direction c. This is represented by thedistances a′ and c′ in FIG. 6. Also, the thin film coating 2 applied tothe substrate experiences, consequently, a comparable expansion,whereupon the electrical resistance of the thin film coating 2 changesdependent on location, or corresponding to the route of the respectivemeasuring path.

FIG. 9 shows a schematic representation of two measuring transducersMT1, MT2 connected for monitoring a process variable in a process, whichis occurring in a pipe or other container, for example. The measuringtransducers MT1, MT2 can be, for example, the resistance temperaturesensors 10 of the invention, however, other sensors can also be used.The firm Endress+Hauser manufacturers an extensive assortment ofresistance thermometers, thermocouples and protective tubes P suitedtherefor.

These sensors are inserted into a protective tube P, which is exposed tothe process. The measuring signals of the respective sensors carried viaconnection lines K1, K2 are supplied to a transmitter unit TU, which asin the case shown in FIG. 9 can be separate from the measuringtransducers MT1 and MT2, respectively. For this purpose the transmitterunit TU has two measurement signal inputs available, which are connectedvia cable to the corresponding connections of the measuring transducers.The measurement signal inputs can be 2 conductor, 3 conductor or 4conductor connections. Thus, the measurement signal inputs can beoptimally matched corresponding to a measurement signal connector of thetemperature sensor element used. For, as already indicated, theresistance of a resistance temperature sensor can be sensed in a 2, 3 or4 conductor measurement.

The measuring signals can be evaluated by the transmitter unit TU and,in given cases, error reports can be output, as for example, in the caseof drift of one of the measuring transducers or both of the measuringtransducers MT1, MT2. On the other hand, the measurement signal, i.e.the measured values collected, which are used for conditioning and/orfurther processing, can be selected temperature dependently.

Especially, a high accuracy of the measuring point, above all however,of the measuring device composed of measuring transducers MT1, MT2 andtransmitter unit TU, can be achieved by so-called sensor transmittermatching. For such purpose, the output of a resistance temperaturesensor 10 is linearized. This can be accomplished, for example, usingthe Callender-van Dusen equation:

R _(T) =R ₀(1+A·T+B·T ²+(T−100)·C·T ³)

where T is the temperature, R_(T) the measured ohmic resistance, R₀ theohmic resistance at 0° C. The coefficients A, B, C serve for matchingthe temperature sensor elements and the transmitter unit TU. In suchcase, a first set of coefficients A, B, C for matching the firsttemperature sensor element and a second set of coefficients A′, B′, C′for matching the second temperature sensor element of a resistancetemperature sensor 10 of the invention can be provided. The coefficientscan be ascertained during a calibration of the temperature sensorelements or the resistance temperature sensor 10 and can be stored, forexample, in the transmitter unit TU.

In contrast with FIG. 9, FIG. 10 shows only one measuring transducer MT.The measuring transducer MT has an installed transmitter unit TU, aso-called temperature head transmitter. The temperature head transmitteris, for example, a two conductor measuring device with, for example, twomeasurement inputs and an analog output A. The measuring transducer MTcan use a temperature sensor element, as shown in FIG. 1 for example, ortwo temperature sensor elements, e.g. in the form of the resistancetemperature sensor 10 of the invention shown in FIG. 2. In the case oftwo temperature sensor elements, these can, for example, be redundantlydesigned. Consequently, the two measurement inputs of the transmitterunit TU can serve to connect a measuring transducer MT with a resistancetemperature sensor of the invention, which has two temperature sensorelements. In such case, thus, the first temperature sensor element canbe connected to the first measurement input and the second temperaturesensor element to the second measurement input. In the transmitter unitTU, the measuring signals of the two temperature sensor elements can be(pre-) processed and/or diagnostic functions can already be executed.Thus, the transmitter unit TU, i.e. a temperature head transmitter, forexample, can safely detect a line break, a short circuit, and corrosion,as well as a wiring error. Moreover, the working range of the measuringtransducer MT and the ambient temperature can be monitored by thetransmitter unit TU. Moreover, corrosion of the measuring transducerconnection lines, which serve for the connecting the measuringtransducer MT with the transmitter unit TU, can be monitored, e.g. whenline resistances exceed plausible limits. In such case, for example, acorresponding error report can be output via the analog output, whichuses for example, a 4 to 20 mA output signal or the HART protocol.

Due to the two measurement signal inputs, either of the embodimentsaccording to FIG. 9 or FIG. 10 additionally can be provided with aso-called sensor backup function, which switches to the secondtemperature sensor element and, for example, outputs this via the analogoutput in case the first temperature sensor element fails. Also the twomeasurement inputs can serve to switch to the first and the secondtemperature sensor element, in case the two temperature sensor elementsshould be applied in different temperature or measuring ranges or areprovided for use in different temperatures. As already mentioned, adrift warning can also be provided in the form of an alarm, which isoutput in the case of a deviation, which lies outside a predeterminedlimit value.

Together with the named components, the temperature head transmitterforms the total measuring point for the most varied of applications inthe industrial environment.

FIG. 11 shows a schematic representation of a device architecture, aspresent, for example, in an industrial plant. The measuring device TMT82has, in such case, an analog output O, via which it can communicate, forexample, by means of a 4 to 20 mA electrical current signal and the HARTprotocol. The analog output O is connected by connecting lines to anRN221N active barrier, which supplies the measuring device TMT82 withauxiliary energy and transmits a measurement signal issued by themeasuring device TMT82, for example, to a process control system PLC.

Other communication interfaces, such as a Bluetooth interface B and aCommubox signal converter, can also be connected to the active barrierRN221N, to enable communication, respectively, with an SFX100 handhelddevice and laptop computer C connected to the feed separator RN221N.

As presented in FIG. 11, a Bluetooth interface B can be connected to theRN221N active barrier, so that a SFX100 handheld device can becommunicated with, which has, for example, a display unit D2, on whichcan be displayed especially measured values and/or other processrelevant data.

Additionally, instead of, or in parallel with, the Bluetooth interface,a further communication interface, such as, for example, a Commuboxsignal converter of the firm, Endress+Hauser, can be connected. TheCommubox signal converter, for example, is an intrinsically safecommunication interface for transmitter units TU for converting HARTsignals to USB signals and thereby enabling communication with acomputer C. In turn, such a computer C can run a process diagnosisand/or maintenance program, such as, for example, the Fieldcare softwareof the firm, Endress+Hauser. Of course, measured values and/or processrelevant data can also then be displayed on this computer, especially onthe display unit D1.

LIST OF REFERENCE CHARACTERS

1 substrate

2 coating material

3 substrate cover

4 inerting, embedding material

5 double contacting

6 first pair of contacts

7 second pair of contacts

a first expansion direction

c second expansion direction

TU transmitter unit

MT measuring transducer

MT1 first measuring transducer

MT2 second measuring transducer

SFX 100 handheld device

B Bluetooth interface

Commubox USB interface

C computer

PLC process control system

RN221N active barrier

D1 first display unit

D2 second display unit

Fieldcare diagnosis/maintenance program

O analog output

P protective tube

72 conductive traces

32 conductive traces

31 electrical contacts

42 conductive traces

a′, c′ length change in expansion direction a, c

10 resistance temperature sensor

1-15. (canceled)
 16. A temperature measuring device, comprising: twomeasuring transducers for monitoring a process variable in a process,which is occurring in a pipe or other container, each measuringtransducer comprises a resistance temperature sensor; and the measuringsignals of the respective resistance temperature sensors are carried viaconnection lines and are supplied to a transmitter unit, wherein: saidtransmitter unit has two measurement signal inputs available, which areconnected via cable to the corresponding connections of the measuringtransducers; and the measurement signals are evaluated by saidtransmitter unit and, in given cases, error reports are output in thecase of drift of one of said measuring transducers or both of saidmeasuring transducers.
 17. The temperature measuring device according toclaim 16, wherein: a high accuracy of said temperature measuring deviceis achieved by sensor-transmitter-matching, e.g. by using theCallendar-van-Dusen equation.
 18. The temperature measuring deviceaccording to claim 17, wherein: the coefficients (A, B, C) serve formatching the respective temperature sensor elements and the transmitterunit, i.e. a first set of coefficients (A, B, C) for matching the firsttemperature sensor element and a second set of coefficients (A′, B′, C′)for matching the second temperature sensor element.
 19. A measuringdevice according to claim 18, wherein: the coefficients are ascertainedduring a calibration of the temperature sensor elements and are storedin the transmitter unit.
 20. A temperature head transmitter; wherein:the temperature head transmitter is a two conductor measuring devicewith two measurement inputs and an analog output; the temperature headtransmitter has a build-in transmitter unit, the temperature headtransmitter has a single the measuring transducer with a first and asecond temperature sensor element, the first temperature sensor elementis connected to the first measurement input and the second temperaturesensor element to the second measurement input of the temperature headtransmitter; and in the transmitter unit, the measuring signals of thetwo temperature sensor elements are be (pre-)processed and/or diagnosticfunctions using the measuring signals are executed.
 21. A temperaturehead transmitter according to claim 20, wherein: the temperature headtransmitter detects a line break, a short circuit, and a corrosion, aswell as a wiring error respectively.
 22. The temperature headtransmitter according to claim 20, wherein: the working range of themeasuring transducer and the ambient temperature is monitored by saidtransmitter unit.
 23. A temperature head transmitter according to claim20, wherein: corrosion of said measuring transducer connection lines,which serve for the connecting said measuring transducer with saidtransmitter unit, are monitored, and in case the line resistances exceedplausible limits a corresponding error report can be output via theanalog output.